A Roman Catholic priest and a Marist brother are facing charges involving sexual abuse of students at a parochial preparatory school in the Bronx with which they were formerly connected, the Bronx District Attorney, Paul T. Gentile, announced yesterday.

He said the indictment resulted from an investigation by the Federal and city authorities begun last July because of complaints from lay teachers at the school, Mount St. Michael Academy. The school is in the Wakefield section, its large campus bordering on the Mount Vernon city line.

Mr. Gentile identified one of the men as the Rev. Bernard Lynch, 41 years old, and a former campus chaplain. An Irish national, he is believed to be somewhere in Ireland. He was indicted on two counts of first-degree sexual abuse, two counts of third-degree sexual abuse and one count of endangering the welfare of a child.

Mr. Gentile said Brother Timothy Brady, 41, the acting principal when the incidents are said to have occurred, was charged with one count each of third-degree sexual abuse and endangering the welfare of a child. Brother Brady "was removed last summer" to a Marist retreat in Arizona, Mr. Gentile said.

Brother Anthony Iazzetti, principal of the school, said school officials fully cooperated with investigators from the joint Sexual Exploited Child Task Force. He called the indictments by the Bronx County grand jury "a source of grave concern."

School officials said Brother Brady had left at the end of the 1986-87 academic year but remained a member of the Marist Brothers religious order, which operates the school.